Glucose-6-phosphate dehydrogenase assays

ABSTRACT

Aspects of embodiments may include methods for automated enzymatic detection of glucose-6-phosphate dehydrogenase (G6PD) activity. Aspects of embodiments may include methods for enzymatic detection of G6PD activity in droplets in oil. Aspects of embodiments may include a system including a droplet actuator. Aspects of embodiments may include a treatment method.

1 BACKGROUND

A droplet actuator typically includes one or more substrates configured to form a surface or gap for conducting droplet operations. The one or more substrates establish a droplet operations surface or gap for conducting droplet operations and may also include electrodes arranged to conduct the droplet operations. The droplet operations substrate or the gap between the substrates may be coated or filled with a filler fluid that is immiscible with the liquid that forms the droplets.

Droplet actuators are used in a variety of applications, including molecular diagnostic assays, such as enzymatic assays. In one application, enzymatic assays are used as part of a routine testing process to test newborn infants for various genetic disorders. For example, enzymatic assays may be used to test for deficiencies in glucose-6-phosphate dehydrogenase (G6PD) activity. Currently, the most commonly used enzymatic assay for G6PD deficiency is a rapid qualitative fluorescent spot test detecting the generation of NADPH from NADP. The test is positive if the blood spot fails to fluoresce under ultraviolet light, i.e., there is marked deficiency or lack of G6PD enzyme activity when no fluorescence is observed. However, this assay is a qualitative assay that requires highly trained personnel to visually “read” the sample for fluorescence or lack of fluorescence. Therefore, there is a need for new approaches to G6PD deficiency testing that is less prone to error and that is more readily available.

2 DEFINITIONS

As used herein, the following terms have the meanings indicated.

“Activate,” with reference to one or more electrodes, means affecting a change in the electrical state of the one or more electrodes which, in the presence of a droplet, results in a droplet operation. Activation of an electrode can be accomplished using alternating or direct current. Any suitable voltage may be used. For example, an electrode may be activated using a voltage which is greater than about 150 V, or greater than about 200 V, or greater than about 250 V, or from about 275 V to about 1000 V, or about 300 V. Where alternating current is used, any suitable frequency may be employed. For example, an electrode may be activated using alternating current having a frequency from about 1 Hz to about 10 MHz, or from about 10 Hz to about 60 Hz, or from about 20 Hz to about 40 Hz, or about 30 Hz.

“Bead,” with respect to beads on a droplet actuator, means any bead or particle that is capable of interacting with a droplet on or in proximity with a droplet actuator. Beads may be any of a wide variety of shapes, such as spherical, generally spherical, egg shaped, disc shaped, cubical, amorphous and other three dimensional shapes. The bead may, for example, be capable of being subjected to a droplet operation in a droplet on a droplet actuator or otherwise configured with respect to a droplet actuator in a manner which permits a droplet on the droplet actuator to be brought into contact with the bead on the droplet actuator and/or off the droplet actuator. Beads may be provided in a droplet, in a droplet operations gap, or on a droplet operations surface. Beads may be provided in a reservoir that is external to a droplet operations gap or situated apart from a droplet operations surface, and the reservoir may be associated with a flow path that permits a droplet including the beads to be brought into a droplet operations gap or into contact with a droplet operations surface. Beads may be manufactured using a wide variety of materials, including for example, resins, and polymers. The beads may be any suitable size, including for example, microbeads, microparticles, nanobeads and nanoparticles. In some cases, beads are magnetically responsive; in other cases beads are not significantly magnetically responsive. For magnetically responsive beads, the magnetically responsive material may constitute substantially all of a bead, a portion of a bead, or only one component of a bead. The remainder of the bead may include, among other things, polymeric material, coatings, and moieties which permit attachment of an assay reagent. Examples of suitable beads include flow cytometry microbeads, polystyrene microparticles and nanoparticles, functionalized polystyrene microparticles and nanoparticles, coated polystyrene microparticles and nanoparticles, silica microbeads, fluorescent microspheres and nanospheres, functionalized fluorescent microspheres and nanospheres, coated fluorescent microspheres and nanospheres, color dyed microparticles and nanoparticles, magnetic microparticles and nanoparticles, superparamagnetic microparticles and nanoparticles (e.g., DYNABEADS® particles, available from Invitrogen Group, Carlsbad, Calif.), fluorescent microparticles and nanoparticles, coated magnetic microparticles and nanoparticles, ferromagnetic microparticles and nanoparticles, coated ferromagnetic microparticles and nanoparticles, and those described in U.S. Patent Publication Nos. 20050260686, entitled “Multiplex flow assays preferably with magnetic particles as solid phase,” published on Nov. 24, 2005; 20030132538, entitled “Encapsulation of discrete quanta of fluorescent particles,” published on Jul. 17, 2003; 20050118574, entitled “Multiplexed Analysis of Clinical Specimens Apparatus and Method,” published on Jun. 2, 2005; 20050277197. Entitled “Microparticles with Multiple Fluorescent Signals and Methods of Using Same,” published on Dec. 15, 2005; 20060159962, entitled “Magnetic Microspheres for use in Fluorescence-based Applications,” published on Jul. 20, 2006; the entire disclosures of which are incorporated herein by reference for their teaching concerning beads and magnetically responsive materials and beads. Beads may be pre-coupled with a biomolecule or other substance that is able to bind to and form a complex with a biomolecule. Beads may be pre-coupled with an antibody, protein or antigen, DNA/RNA probe or any other molecule with an affinity for a desired target. Examples of droplet actuator techniques for immobilizing magnetically responsive beads and/or non-magnetically responsive beads and/or conducting droplet operations protocols using beads are described in U.S. patent application Ser. No. 11/639,566, entitled “Droplet-Based Particle Sorting,” filed on Dec. 15, 2006; U.S. Patent Application No. 61/039,183, entitled “Multiplexing Bead Detection in a Single Droplet,” filed on Mar. 25, 2008; U.S. Patent Application No. 61/047,789, entitled “Droplet Actuator Devices and Droplet Operations Using Beads,” filed on Apr. 25, 2008; U.S. Patent Application No. 61/086,183, entitled “Droplet Actuator Devices and Methods for Manipulating Beads,” filed on Aug. 5, 2008; International Patent Application No. PCT/US2008/053545, entitled “Droplet Actuator Devices and Methods Employing Magnetic Beads,” filed on Feb. 11, 2008; International Patent Application No. PCT/US2008/058018, entitled “Bead-based Multiplexed Analytical Methods and Instrumentation,” filed on Mar. 24, 2008; International Patent Application No. PCT/US2008/058047, “Bead Sorting on a Droplet Actuator,” filed on Mar. 23, 2008; and International Patent Application No. PCT/US2006/047486, entitled “Droplet-based Biochemistry,” filed on Dec. 11, 2006; the entire disclosures of which are incorporated herein by reference. Bead characteristics may be employed in the multiplexing aspects of the invention. Examples of beads having characteristics suitable for multiplexing, as well as methods of detecting and analyzing signals emitted from such beads, may be found in U.S. Patent Publication No. 20080305481, entitled “Systems and Methods for Multiplex Analysis of PCR in Real Time,” published on Dec. 11, 2008; U.S. Patent Publication No. 20080151240, “Methods and Systems for Dynamic Range Expansion,” published on Jun. 26, 2008; U.S. Patent Publication No. 20070207513, entitled “Methods, Products, and Kits for Identifying an Analyte in a Sample,” published on Sep. 6, 2007; U.S. Patent Publication No. 20070064990, entitled “Methods and Systems for Image Data Processing,” published on Mar. 22, 2007; U.S. Patent Publication No. 20060159962, entitled “Magnetic Microspheres for use in Fluorescence-based Applications,” published on Jul. 20, 2006; U.S. Patent Publication No. 20050277197, entitled “Microparticles with Multiple Fluorescent Signals and Methods of Using Same,” published on Dec. 15, 2005; and U.S. Patent Publication No. 20050118574, entitled “Multiplexed Analysis of Clinical Specimens Apparatus and Method,” published on Jun. 2, 2005.

“Droplet” means a volume of liquid on a droplet actuator. Typically, a droplet is at least partially bounded by a filler fluid. For example, a droplet may be completely surrounded by a filler fluid or may be bounded by filler fluid and one or more surfaces of the droplet actuator. As another example, a droplet may be bounded by filler fluid, one or more surfaces of the droplet actuator, and/or the atmosphere. As yet another example, a droplet may be bounded by filler fluid and the atmosphere. Droplets may, for example, be aqueous or non-aqueous or may be mixtures or emulsions including aqueous and non-aqueous components. Droplets may take a wide variety of shapes; nonlimiting examples include generally disc shaped, slug shaped, truncated sphere, ellipsoid, spherical, partially compressed sphere, hemispherical, ovoid, cylindrical, combinations of such shapes, and various shapes formed during droplet operations, such as merging or splitting or formed as a result of contact of such shapes with one or more surfaces of a droplet actuator. For examples of droplet fluids that may be subjected to droplet operations using the approach of the invention, see International Patent Application No. PCT/US 06/47486, entitled, “Droplet-Based Biochemistry,” filed on Dec. 11, 2006. In various embodiments, a droplet may include a biological sample, such as whole blood, lymphatic fluid, serum, plasma, sweat, tear, saliva, sputum, cerebrospinal fluid, amniotic fluid, seminal fluid, vaginal excretion, serous fluid, synovial fluid, pericardial fluid, peritoneal fluid, pleural fluid, transudates, exudates, cystic fluid, bile, urine, gastric fluid, intestinal fluid, fecal samples, liquids containing single or multiple cells, liquids containing organelles, fluidized tissues, fluidized organisms, liquids containing multi-celled organisms, biological swabs and biological washes. Moreover, a droplet may include a reagent, such as water, deionized water, saline solutions, acidic solutions, basic solutions, detergent solutions and/or buffers. Other examples of droplet contents include reagents, such as a reagent for a biochemical protocol, such as a nucleic acid amplification protocol, an affinity-based assay protocol, an enzymatic assay protocol, a sequencing protocol, and/or a protocol for analyses of biological fluids. A droplet may include one or more beads.

“Droplet Actuator” means a device for manipulating droplets. For examples of droplet actuators, see Pamula et al., U.S. Pat. No. 6,911,132, entitled “Apparatus for Manipulating Droplets by Electrowetting-Based Techniques,” issued on Jun. 28, 2005; Pamula et al., U.S. patent application Ser. No. 11/343,284, entitled “Apparatuses and Methods for Manipulating Droplets on a Printed Circuit Board,” filed on filed on Jan. 30, 2006; Pollack et al., International Patent Application No. PCT/US2006/047486, entitled “Droplet-Based Biochemistry,” filed on Dec. 11, 2006; Shenderov, U.S. Pat. No. 6,773,566, entitled “Electrostatic Actuators for Microfluidics and Methods for Using Same,” issued on Aug. 10, 2004 and U.S. Pat. No. 6,565,727, entitled “Actuators for Microfluidics Without Moving Parts,” issued on Jan. 24, 2000; Kim and/or Shah et al., U.S. patent application Ser. No. 10/343,261, entitled “Electrowetting-driven Micropumping,” filed on Jan. 27, 2003, Ser. No. 11/275,668, entitled “Method and Apparatus for Promoting the Complete Transfer of Liquid Drops from a Nozzle,” filed on Jan. 23, 2006, Ser. No. 11/460,188, entitled “Small Object Moving on Printed Circuit Board,” filed on Jan. 23, 2006, Ser. No. 12/465,935, entitled “Method for Using Magnetic Particles in Droplet Microfluidics,” filed on May 14, 2009, and Ser. No. 12/513,157, entitled “Method and Apparatus for Real-time Feedback Control of Electrical Manipulation of Droplets on Chip,” filed on Apr. 30, 2009; Velev, U.S. Pat. No. 7,547,380, entitled “Droplet Transportation Devices and Methods Having a Fluid Surface,” issued on Jun. 16, 2009; Sterling et al., U.S. Pat. No. 7,163,612, entitled “Method, Apparatus and Article for Microfluidic Control via Electrowetting, for Chemical, Biochemical and Biological Assays and the Like,” issued on Jan. 16, 2007; Becker and Gascoyne et al., U.S. Pat. No. 7,641,779, entitled “Method and Apparatus for Programmable fluidic Processing,” issued on Jan. 5, 2010, and U.S. Pat. No. 6,977,033, entitled “Method and Apparatus for Programmable fluidic Processing,” issued on Dec. 20, 2005; Decre et al., U.S. Pat. No. 7,328,979, entitled “System for Manipulation of a Body of Fluid,” issued on Feb. 12, 2008; Yamakawa et al., U.S. Patent Pub. No. 20060039823, entitled “Chemical Analysis Apparatus,” published on Feb. 23, 2006; Wu, International Patent Pub. No. WO/2009/003184, entitled “Digital Microfluidics Based Apparatus for Heat-exchanging Chemical Processes,” published on Dec. 31, 2008; Fouillet et al., U.S. Patent Pub. No. 20090192044, entitled “Electrode Addressing Method,” published on Jul. 30, 2009; Fouillet et al., U.S. Pat. No. 7,052,244, entitled “Device for Displacement of Small Liquid Volumes Along a Micro-catenary Line by Electrostatic Forces,” issued on May 30, 2006; Marchand et al., U.S. Patent Pub. No. 20080124252, entitled “Droplet Microreactor,” published on May 29, 2008; Adachi et al., U.S. Patent Pub. No. 20090321262, entitled “Liquid Transfer Device,” published on Dec. 31, 2009; Roux et al., U.S. Patent Pub. No. 20050179746, entitled “Device for Controlling the Displacement of a Drop Between two or Several Solid Substrates,” published on Aug. 18, 2005; Dhindsa et al., “Virtual Electrowetting Channels: Electronic Liquid Transport with Continuous Channel Functionality,” Lab Chip, 10:832-836 (2010); the entire disclosures of which are incorporated herein by reference, along with their priority documents. Certain droplet actuators will include one or more substrates arranged with a droplet operations gap therebetween and electrodes associated with (e.g., layered on, attached to, and/or embedded in) the one or more substrates and arranged to conduct one or more droplet operations. For example, certain droplet actuators will include a base (or bottom) substrate, droplet operations electrodes associated with the substrate, one or more dielectric layers atop the substrate and/or electrodes, and optionally one or more hydrophobic layers atop the substrate, dielectric layers and/or the electrodes forming a droplet operations surface. A top substrate may also be provided, which is separated from the droplet operations surface by a gap, commonly referred to as a droplet operations gap. Various electrode arrangements on the top and/or bottom substrates are discussed in the above-referenced patents and applications and certain novel electrode arrangements are discussed in the description of the invention. During droplet operations it is preferred that droplets remain in continuous contact or frequent contact with a ground or reference electrode. A ground or reference electrode may be associated with the top substrate facing the gap, the bottom substrate facing the gap, in the gap. Where electrodes are provided on both substrates, electrical contacts for coupling the electrodes to a droplet actuator instrument for controlling or monitoring the electrodes may be associated with one or both plates. In some cases, electrodes on one substrate are electrically coupled to the other substrate so that only one substrate is in contact with the droplet actuator. In one embodiment, a conductive material (e.g., an epoxy, such as MASTER BOND™ Polymer System EP79, available from Master Bond, Inc., Hackensack, N.J.) provides the electrical connection between electrodes on one substrate and electrical paths on the other substrates, e.g., a ground electrode on a top substrate may be coupled to an electrical path on a bottom substrate by such a conductive material. Where multiple substrates are used, a spacer may be provided between the substrates to determine the height of the gap therebetween and define dispensing reservoirs. The spacer height may, for example, be from about 5 μm to about 600 μm, or about 100 μm to about 400 μm, or about 200 μm to about 350 μm, or about 250 μm to about 300 μm, or about 275 μm. The spacer may, for example, be formed of a layer of projections form the top or bottom substrates, and/or a material inserted between the top and bottom substrates. One or more openings may be provided in the one or more substrates for forming a fluid path through which liquid may be delivered into the droplet operations gap. The one or more openings may in some cases be aligned for interaction with one or more electrodes, e.g., aligned such that liquid flowed through the opening will come into sufficient proximity with one or more droplet operations electrodes to permit a droplet operation to be effected by the droplet operations electrodes using the liquid. The base (or bottom) and top substrates may in some cases be formed as one integral component. One or more reference electrodes may be provided on the base (or bottom) and/or top substrates and/or in the gap. Examples of reference electrode arrangements are provided in the above referenced patents and patent applications. In various embodiments, the manipulation of droplets by a droplet actuator may be electrode mediated, e.g., electrowetting mediated or dielectrophoresis mediated or Coulombic force mediated. Examples of other techniques for controlling droplet operations that may be used in the droplet actuators of the invention include using devices that induce hydrodynamic fluidic pressure, such as those that operate on the basis of mechanical principles (e.g. external syringe pumps, pneumatic membrane pumps, vibrating membrane pumps, vacuum devices, centrifugal forces, piezoelectric/ultrasonic pumps and acoustic forces); electrical or magnetic principles (e.g. electroosmotic flow, electrokinetic pumps, ferrofluidic plugs, electrohydrodynamic pumps, attraction or repulsion using magnetic forces and magnetohydrodynamic pumps); thermodynamic principles (e.g. gas bubble generation/phase-change-induced volume expansion); other kinds of surface-wetting principles (e.g. electrowetting, and optoelectrowetting, as well as chemically, thermally, structurally and radioactively induced surface-tension gradients); gravity; surface tension (e.g., capillary action); electrostatic forces (e.g., electroosmotic flow); centrifugal flow (substrate disposed on a compact disc and rotated); magnetic forces (e.g., oscillating ions causes flow); magnetohydrodynamic forces; and vacuum or pressure differential. In certain embodiments, combinations of two or more of the foregoing techniques may be employed to conduct a droplet operation in a droplet actuator of the invention. Similarly, one or more of the foregoing may be used to deliver liquid into a droplet operations gap, e.g., from a reservoir in another device or from an external reservoir of the droplet actuator (e.g., a reservoir associated with a droplet actuator substrate and a flow path from the reservoir into the droplet operations gap). Droplet operations surfaces of certain droplet actuators of the invention may be made from hydrophobic materials or may be coated or treated to make them hydrophobic. For example, in some cases some portion or all of the droplet operations surfaces may be derivatized with low surface-energy materials or chemistries, e.g., by deposition or using in situ synthesis using compounds such as poly- or per-fluorinated compounds in solution or polymerizable monomers. Examples include TEFLON® AF (available from DuPont, Wilmington, Del.), members of the cytop family of materials, coatings in the FLUOROPEL® family of hydrophobic and superhydrophobic coatings (available from Cytonix Corporation, Beltsville, Md.), silane coatings, fluorosilane coatings, hydrophobic phosphonate derivatives (e.g., those sold by Aculon, Inc), and NOVEC™ electronic coatings (available from 3M Company, St. Paul, Minn.), other fluorinated monomers for plasma-enhanced chemical vapor deposition (PECVD), and organosiloxane (e.g., SiOC) for PECVD. In some cases, the droplet operations surface may include a hydrophobic coating having a thickness ranging from about 10 nm to about 1,000 nm. Moreover, in some embodiments, the top substrate of the droplet actuator includes an electrically conducting organic polymer, which is then coated with a hydrophobic coating or otherwise treated to make the droplet operations surface hydrophobic. For example, the electrically conducting organic polymer that is deposited onto a plastic substrate may be poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS). Other examples of electrically conducting organic polymers and alternative conductive layers are described in Pollack et al., International Patent Application No. PCT/US2010/040705, entitled “Droplet Actuator Devices and Methods,” the entire disclosure of which is incorporated herein by reference. One or both substrates may be fabricated using a printed circuit board (PCB), glass, indium tin oxide (ITO)-coated glass, and/or semiconductor materials as the substrate. When the substrate is ITO-coated glass, the ITO coating is preferably a thickness in the range of about 20 to about 200 nm, preferably about 50 to about 150 nm, or about 75 to about 125 nm, or about 100 nm. In some cases, the top and/or bottom substrate includes a PCB substrate that is coated with a dielectric, such as a polyimide dielectric, which may in some cases also be coated or otherwise treated to make the droplet operations surface hydrophobic. When the substrate includes a PCB, the following materials are examples of suitable materials: MITSUI™ BN-300 (available from MITSUI Chemicals America, Inc., San Jose Calif.); ARLON™ 11N (available from Arlon, Inc, Santa Ana, Calif.); NELCO® N4000-6 and N5000-30/32 (available from Park Electrochemical Corp., Melville, N.Y.); ISOLA™ FR406 (available from Isola Group, Chandler, Ariz.), especially IS620; fluoropolymer family (suitable for fluorescence detection since it has low background fluorescence); polyimide family; polyester; polyethylene naphthalate; polycarbonate; polyetheretherketone; liquid crystal polymer; cyclo-olefin copolymer (COC); cyclo-olefin polymer (COP); aramid; THERMOUNT® nonwoven aramid reinforcement (available from DuPont, Wilmington, Del.); NOMEX® brand fiber (available from DuPont, Wilmington, Del.); and paper. Various materials are also suitable for use as the dielectric component of the substrate. Examples include: vapor deposited dielectric, such as PARYLENE™ C (especially on glass), PARYLENE™ N, and PARYLENE™ HT (for high temperature, ˜300° C.) (available from Parylene Coating Services, Inc., Katy, Tex.); TEFLON® AF coatings; cytop; soldermasks, such as liquid photoimageable soldermasks (e.g., on PCB) like TAIYO™ PSR4000 series, TAIYO™ PSR and AUS series (available from Taiyo America, Inc. Carson City, Nev.) (good thermal characteristics for applications involving thermal control), and PROBIMER™ 8165 (good thermal characteristics for applications involving thermal control (available from Huntsman Advanced Materials Americas Inc., Los Angeles, Calif.); dry film soldermask, such as those in the VACREL® dry film soldermask line (available from DuPont, Wilmington, Del.); film dielectrics, such as polyimide film (e.g., KAPTON® polyimide film, available from DuPont, Wilmington, Del.), polyethylene, and fluoropolymers (e.g., FEP), polytetrafluoroethylene; polyester; polyethylene naphthalate; cyclo-olefin copolymer (COC); cyclo-olefin polymer (COP); any other PCB substrate material listed above; black matrix resin; and polypropylene. Droplet transport voltage and frequency may be selected for performance with reagents used in specific assay protocols. Design parameters may be varied, e.g., number and placement of on-actuator reservoirs, number of independent electrode connections, size (volume) of different reservoirs, placement of magnets/bead washing zones, electrode size, inter-electrode pitch, and gap height (between top and bottom substrates) may be varied for use with specific reagents, protocols, droplet volumes, etc. In some cases, a substrate of the invention may derivatized with low surface-energy materials or chemistries, e.g., using deposition or in situ synthesis using poly- or per-fluorinated compounds in solution or polymerizable monomers. Examples include TEFLON® AF coatings and FLUOROPEL® coatings for dip or spray coating, other fluorinated monomers for plasma-enhanced chemical vapor deposition (PECVD), and organosiloxane (e.g., SiOC) for PECVD. Additionally, in some cases, some portion or all of the droplet operations surface may be coated with a substance for reducing background noise, such as background fluorescence from a PCB substrate. For example, the noise-reducing coating may include a black matrix resin, such as the black matrix resins available from Toray industries, Inc., Japan. Electrodes of a droplet actuator are typically controlled by a controller or a processor, which is itself provided as part of a system, which may include processing functions as well as data and software storage and input and output capabilities. Reagents may be provided on the droplet actuator in the droplet operations gap or in a reservoir fluidly coupled to the droplet operations gap. The reagents may be in liquid form, e.g., droplets, or they may be provided in a reconstitutable form in the droplet operations gap or in a reservoir fluidly coupled to the droplet operations gap. Reconstitutable reagents may typically be combined with liquids for reconstitution. An example of reconstitutable reagents suitable for use with the invention includes those described in Meathrel, et al., U.S. Pat. No. 7,727,466, entitled “Disintegratable films for diagnostic devices,” granted on Jun. 1, 2010.

“Droplet operation” means any manipulation of a droplet on a droplet actuator. A droplet operation may, for example, include: loading a droplet into the droplet actuator; dispensing one or more droplets from a source droplet; splitting, separating or dividing a droplet into two or more droplets; transporting a droplet from one location to another in any direction; merging or combining two or more droplets into a single droplet; diluting a droplet; mixing a droplet; agitating a droplet; deforming a droplet; retaining a droplet in position; incubating a droplet; heating a droplet; vaporizing a droplet; cooling a droplet; disposing of a droplet; transporting a droplet out of a droplet actuator; other droplet operations described herein; and/or any combination of the foregoing. The terms “merge,” “merging,” “combine,” “combining” and the like are used to describe the creation of one droplet from two or more droplets. It should be understood that when such a term is used in reference to two or more droplets, any combination of droplet operations that are sufficient to result in the combination of the two or more droplets into one droplet may be used. For example, “merging droplet A with droplet B,” can be achieved by transporting droplet A into contact with a stationary droplet B, transporting droplet B into contact with a stationary droplet A, or transporting droplets A and B into contact with each other. The terms “splitting,” “separating” and “dividing” are not intended to imply any particular outcome with respect to volume of the resulting droplets (i.e., the volume of the resulting droplets can be the same or different) or number of resulting droplets (the number of resulting droplets may be 2, 3, 4, 5 or more). The term “mixing” refers to droplet operations which result in more homogenous distribution of one or more components within a droplet. Examples of “loading” droplet operations include microdialysis loading, pressure assisted loading, robotic loading, passive loading, and pipette loading. Droplet operations may be electrode-mediated. In some cases, droplet operations are further facilitated by the use of hydrophilic and/or hydrophobic regions on surfaces and/or by physical obstacles. For examples of droplet operations, see the patents and patent applications cited above under the definition of “droplet actuator.” Impedance or capacitance sensing or imaging techniques may sometimes be used to determine or confirm the outcome of a droplet operation. Examples of such techniques are described in Sturmer et al., International Patent Pub. No. WO/2008/101194, entitled “Capacitance Detection in a Droplet Actuator,” published on Aug. 21, 2008, the entire disclosure of which is incorporated herein by reference. Generally speaking, the sensing or imaging techniques may be used to confirm the presence or absence of a droplet at a specific electrode. For example, the presence of a dispensed droplet at the destination electrode following a droplet dispensing operation confirms that the droplet dispensing operation was effective. Similarly, the presence of a droplet at a detection spot at an appropriate step in an assay protocol may confirm that a previous set of droplet operations has successfully produced a droplet for detection. Droplet transport time can be quite fast. For example, in various embodiments, transport of a droplet from one electrode to the next may exceed about 1 sec, or about 0.1 sec, or about 0.01 sec, or about 0.001 sec. In one embodiment, the electrode is operated in AC mode but is switched to DC mode for imaging. It is helpful for conducting droplet operations for the footprint area of droplet to be similar to electrowetting area; in other words, 1×-, 2×-3×-droplets are usefully controlled operated using 1, 2, and 3 electrodes, respectively. If the droplet footprint is greater than the number of electrodes available for conducting a droplet operation at a given time, the difference between the droplet size and the number of electrodes should typically not be greater than 1; in other words, a 2× droplet is usefully controlled using 1 electrode and a 3× droplet is usefully controlled using 2 electrodes. When droplets include beads, it is useful for droplet size to be equal to the number of electrodes controlling the droplet, e.g., transporting the droplet.

“Filler fluid” means a fluid associated with a droplet operations substrate of a droplet actuator, which fluid is sufficiently immiscible with a droplet phase to render the droplet phase subject to electrode-mediated droplet operations. For example, the droplet operations gap of a droplet actuator is typically filled with a filler fluid. The filler fluid may, for example, be or include a low-viscosity oil, such as silicone oil or hexadecane filler fluid. The filler fluid may be or include a halogenated oil, such as a fluorinated or perfluorinated oil. The filler fluid may fill the entire gap of the droplet actuator or may coat one or more surfaces of the droplet actuator. Filler fluids may be conductive or non-conductive. Filler fluids may be selected to improve droplet operations and/or reduce loss of reagent or target substances from droplets, improve formation of microdroplets, reduce cross contamination between droplets, reduce contamination of droplet actuator surfaces, reduce degradation of droplet actuator materials, etc. For example, filler fluids may be selected for compatibility with droplet actuator materials. As an example, fluorinated filler fluids may be usefully employed with fluorinated surface coatings. Fluorinated filler fluids are useful to reduce loss of lipophilic compounds, such as umbelliferone substrates like 6-hexadecanoylamido-4-methylumbelliferone substrates (e.g., for use in Krabbe, Niemann-Pick, or other assays); other umbelliferone substrates are described in U.S. Patent Pub. No. 20110118132, published on May 19, 2011, the entire disclosure of which is incorporated herein by reference. Examples of suitable fluorinated oils include those in the Galden line, such as Galden HT170 (bp=170° C., viscosity=1.8 cSt, density=1.77), Galden HT200 (bp=200 C, viscosity=2.4 cSt, d=1.79), Galden HT230 (bp=230 C, viscosity=4.4 cSt, d=1.82) (all from Solvay Solexis); those in the Novec line, such as Novec 7500 (bp=128 C, viscosity=0.8 cSt, d=1.61), Fluorinert FC-40 (bp=155° C., viscosity=1.8 cSt, d=1.85), Fluorinert FC-43 (bp=174° C., viscosity=2.5 cSt, d=1.86) (both from 3M). In general, selection of perfluorinated filler fluids is based on kinematic viscosity (<7 cSt is preferred, but not required), and on boiling point (>150° C. is preferred, but not required, for use in DNA/RNA-based applications (PCR, etc.)). Filler fluids may, for example, be doped with surfactants or other additives. For example, additives may be selected to improve droplet operations and/or reduce loss of reagent or target substances from droplets, formation of microdroplets, cross contamination between droplets, contamination of droplet actuator surfaces, degradation of droplet actuator materials, etc. Composition of the filler fluid, including surfactant doping, may be selected for performance with reagents used in the specific assay protocols and effective interaction or non-interaction with droplet actuator materials. Examples of filler fluids and filler fluid formulations suitable for use with the invention are provided in Srinivasan et al, International Patent Pub. Nos. WO/2010/027894, entitled “Droplet Actuators, Modified Fluids and Methods,” published on Mar. 11, 2010, and WO/2009/021173, entitled “Use of Additives for Enhancing Droplet Operations,” published on Feb. 12, 2009; Sista et al., International Patent Pub. No. WO/2008/098236, entitled “Droplet Actuator Devices and Methods Employing Magnetic Beads,” published on Aug. 14, 2008; and Monroe et al., U.S. Patent Publication No. 20080283414, entitled “Electrowetting Devices,” filed on May 17, 2007; the entire disclosures of which are incorporated herein by reference, as well as the other patents and patent applications cited herein. Fluorinated oils may in some cases be doped with fluorinated surfactants, e.g., Zonyl FSO-100 (Sigma-Aldrich) and/or others.

“Immobilize” with respect to magnetically responsive beads, means that the beads are substantially restrained in position in a droplet or in filler fluid on a droplet actuator. For example, in one embodiment, immobilized beads are sufficiently restrained in position in a droplet to permit execution of a droplet splitting operation, yielding one droplet with substantially all of the beads and one droplet substantially lacking in the beads.

“Magnetically responsive” means responsive to a magnetic field. “Magnetically responsive beads” include or are composed of magnetically responsive materials. Examples of magnetically responsive materials include paramagnetic materials, ferromagnetic materials, ferrimagnetic materials, and metamagnetic materials. Examples of suitable paramagnetic materials include iron, nickel, and cobalt, as well as metal oxides, such as Fe3O4, BaFe12O19, CoO, NiO, Mn2O3, Cr2O3, and CoMnP.

“Reservoir” means an enclosure or partial enclosure configured for holding, storing, or supplying liquid. A droplet actuator system of the invention may include on-cartridge reservoirs and/or off-cartridge reservoirs. On-cartridge reservoirs may be (1) on-actuator reservoirs, which are reservoirs in the droplet operations gap or on the droplet operations surface; (2) off-actuator reservoirs, which are reservoirs on the droplet actuator cartridge, but outside the droplet operations gap, and not in contact with the droplet operations surface; or (3) hybrid reservoirs which have on-actuator regions and off-actuator regions. An example of an off-actuator reservoir is a reservoir in the top substrate. An off-actuator reservoir is typically in fluid communication with an opening or flow path arranged for flowing liquid from the off-actuator reservoir into the droplet operations gap, such as into an on-actuator reservoir. An off-cartridge reservoir may be a reservoir that is not part of the droplet actuator cartridge at all, but which flows liquid to some portion of the droplet actuator cartridge. For example, an off-cartridge reservoir may be part of a system or docking station to which the droplet actuator cartridge is coupled during operation. Similarly, an off-cartridge reservoir may be a reagent storage container or syringe which is used to force fluid into an on-cartridge reservoir or into a droplet operations gap. A system using an off-cartridge reservoir will typically include a fluid passage means whereby liquid may be transferred from the off-cartridge reservoir into an on-cartridge reservoir or into a droplet operations gap.

“Transporting into the magnetic field of a magnet,” “transporting towards a magnet,” and the like, as used herein to refer to droplets and/or magnetically responsive beads within droplets, is intended to refer to transporting into a region of a magnetic field capable of substantially attracting magnetically responsive beads in the droplet. Similarly, “transporting away from a magnet or magnetic field,” “transporting out of the magnetic field of a magnet,” and the like, as used herein to refer to droplets and/or magnetically responsive beads within droplets, is intended to refer to transporting away from a region of a magnetic field capable of substantially attracting magnetically responsive beads in the droplet, whether or not the droplet or magnetically responsive beads is completely removed from the magnetic field. It will be appreciated that in any of such cases described herein, the droplet may be transported towards or away from the desired region of the magnetic field, and/or the desired region of the magnetic field may be moved towards or away from the droplet. Reference to an electrode, a droplet, or magnetically responsive beads being “within” or “in” a magnetic field, or the like, is intended to describe a situation in which the electrode is situated in a manner which permits the electrode to transport a droplet into and/or away from a desired region of a magnetic field, or the droplet or magnetically responsive beads is/are situated in a desired region of the magnetic field, in each case where the magnetic field in the desired region is capable of substantially attracting any magnetically responsive beads in the droplet. Similarly, reference to an electrode, a droplet, or magnetically responsive beads being “outside of” or “away from” a magnetic field, and the like, is intended to describe a situation in which the electrode is situated in a manner which permits the electrode to transport a droplet away from a certain region of a magnetic field, or the droplet or magnetically responsive beads is/are situated away from a certain region of the magnetic field, in each case where the magnetic field in such region is not capable of substantially attracting any magnetically responsive beads in the droplet or in which any remaining attraction does not eliminate the effectiveness of droplet operations conducted in the region. In various aspects of the invention, a system, a droplet actuator, or another component of a system may include a magnet, such as one or more permanent magnets (e.g., a single cylindrical or bar magnet or an array of such magnets, such as a Halbach array) or an electromagnet or array of electromagnets, to form a magnetic field for interacting with magnetically responsive beads or other components on chip. Such interactions may, for example, include substantially immobilizing or restraining movement or flow of magnetically responsive beads during storage or in a droplet during a droplet operation or pulling magnetically responsive beads out of a droplet.

“Washing” with respect to washing a bead means reducing the amount and/or concentration of one or more substances in contact with the bead or exposed to the bead from a droplet in contact with the bead. The reduction in the amount and/or concentration of the substance may be partial, substantially complete, or even complete. The substance may be any of a wide variety of substances; examples include target substances for further analysis, and unwanted substances, such as components of a sample, contaminants, and/or excess reagent. In some embodiments, a washing operation begins with a starting droplet in contact with a magnetically responsive bead, where the droplet includes an initial amount and initial concentration of a substance. The washing operation may proceed using a variety of droplet operations. The washing operation may yield a droplet including the magnetically responsive bead, where the droplet has a total amount and/or concentration of the substance which is less than the initial amount and/or concentration of the substance. Examples of suitable washing techniques are described in Pamula et al., U.S. Pat. No. 7,439,014, entitled “Droplet-Based Surface Modification and Washing,” granted on Oct. 21, 2008, the entire disclosure of which is incorporated herein by reference.

The terms “top,” “bottom,” “over,” “under,” and “on” are used throughout the description with reference to the relative positions of components of the droplet actuator, such as relative positions of top and bottom substrates of the droplet actuator. It will be appreciated that the droplet actuator is functional regardless of its orientation in space.

When a liquid in any form (e.g., a droplet or a continuous body, whether moving or stationary) is described as being “on”, “at”, or “over” an electrode, array, matrix or surface, such liquid could be either in direct contact with the electrode/array/matrix/surface, or could be in contact with one or more layers or films that are interposed between the liquid and the electrode/array/matrix/surface. In one example, filler fluid can be considered as a film between such liquid and the electrode/array/matrix/surface.

When a droplet is described as being “on” or “loaded on” a droplet actuator, it should be understood that the droplet is arranged on the droplet actuator in a manner which facilitates using the droplet actuator to conduct one or more droplet operations on the droplet, the droplet is arranged on the droplet actuator in a manner which facilitates sensing of a property of or a signal from the droplet, and/or the droplet has been subjected to a droplet operation on the droplet actuator.

3 BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a process of reducing NADP⁺ to NADPH to generate a fluorescence signal;

FIG. 2 shows a plot of an example of a reference assay using purified G6PD to evaluate substrate formulation for automated G6PD activity assays;

FIG. 3 shows a plot of an example of a comparison of G6PD enzyme activity assays performed on-bench using fresh-frozen whole blood and DBS extract samples;

FIG. 4 shows a plot of an example of the effect of excitation wavelength on NADPH fluorescence in G6PD enzyme activity assays;

FIG. 5 shows a plot of an example of the effect of hematocrit on NADPH fluorescence in a G6PD assay;

FIG. 6 shows a plot of normal, intermediate and deficient G6PD control samples screened on-bench for G6PD activity;

FIG. 7 illustrates a functional block diagram of an example of a microfluidics system that includes a droplet actuator.

4 DESCRIPTION

Aspects of embodiments includes methods for automated enzymatic detection of glucose-6-phosphate dehydrogenase (G6PD) activity. In one embodiment, the invention provides methods for enzymatic detection of G6PD activity in droplets in oil. The droplet-based method includes, among other things, incubating a droplet in oil, the droplet comprising a substrate liquid and a sample liquid. In various embodiments, the invention may provide methods for conducting G6PD enzymatic activity assays in fresh blood samples, fresh-frozen blood samples, and dried blood spot (DBS) samples.

In one embodiment, the enzymatic assays for G6PD activity are used for newborn testing for G6PD deficiency. Droplet-based enzymatic assays for G6PD activity may be combined with other droplet-based enzymatic assays in a panel of tests for newborn testing. In one example, multiplexed testing of newborns that are at risk for hyperbilirubinemia may include testing for total bilirubin, G6PD deficiency, and congenital hypothyroidism. G6PD deficiency and congenital hypothyroidism are the most common underlying pathological causes for hyperbilirubinemia.

Aspects of embodiments of the invention may include a method of detecting glucose-6-phosphate dehydrogenase activity by preparing a sample, preparing a substrate formulation, mixing and aliquot of prepared sample without an aliquot of substrate formulation, incubating the sample at about 37° C. for a time interval, and detecting glucose-6-phosphate dehydrogenase activity present in the sample.

Aspects of embodiments of the invention may include a method of detecting glucose-6-phosphate dehydrogenase activity by mixing an aliquot of prepared sample with an aliquot of substrate formulation, incubating the sample at about 37° C. for a time interval, and detecting glucose-6-phosphate dehydrogenase activity present in the sample.

Aspects of embodiments of the invention may include a method of detecting glucose-6-phosphate dehydrogenase activity by executing electrowetting-mediated droplet operations using droplets on a droplet microactuator to effect an assay, combining one or more substrate formulation droplets with one or more prepared sample droplets, and generating and detecting a signal which corresponds to the conversion of NADP+ to NADPH in the sample.

In another aspect of an embodiment of the invention, a method of detecting glucose-6-phosphate dehydrogenase activity may include conducting the method on a droplet that is partially or completely surrounded by filler fluid on a droplet actuator.

In still another aspect of an embodiment of the invention, the method may further include providing a microfluidic actuator.

Still another aspect of an embodiment of the invention may include detecting the enzymatic conversion of NADP+ to NADPH.

In another aspect of an embodiment of the invention, a method of detecting glucose-6-phosphate dehydrogenase activity may include reading NADPH fluorescence. Additionally, aspects of embodiments may include methods of reading NADPH fluorescence at 340 nm excitation/460 nm emission.

Aspects of embodiments of the invention may include a substrate formulation having a pH of about 7.8, and includes about 100 mM Tris HCL, about 26 mM maleimide, about 2.6 mM NADP+, about 2.4 mM magnesium chloride, and about 2.0 mM glucose-6-phosphate and methods therewith.

Still another aspect of an embodiment may include a method having at least one time interval from about 0 seconds to about 300 seconds. Still further aspects of embodiments may include a method having at least one time interval from about 0 seconds to about 300 seconds.

Aspects of embodiments may include a biological sample.

Aspects of embodiments may include a sample that includes one or more dried blood spots. Aspects of embodiments may include a sample that includes an aliquot of one or more dried blood spots.

In yet another aspect of an embodiment, the sample may be isolated from a patient less than about 30 days old at the time of sample collection. In yet another aspect of an embodiment, the sample may be isolated from a patient less than about 90 days old at the time of sample collection.

In another aspect of an embodiment, the detecting step may include detecting a signal from a droplet on a droplet microactuator. In yet another aspect of an embodiment, a method is provided that may include measuring a signal that corresponds to glucose-6-phosphate dehydrogenase activity.

In another aspect of an embodiment, a computer readable medium programmed to cause a droplet actuator to perform any of the method steps may be included.

Aspects of embodiments may include a system including a droplet actuator coupled to and controlled by a computer program to cause the droplet actuator to perform any method steps of a method.

Aspects of embodiments may include a treatment method including providing a droplet actuator, sample, and glucose-6-phosphate dehydrogenase substrate formulation, executing electrowetting-mediated droplet operations to detect glucose-6-phosphate dehydrogenase (G6PD) activity in a sample; detecting glucose-6-phosphate dehydrogenase (G6PD) activity; and discontinuing the administration of one or more of Dapsone, Flutamide (Eulexin), Mafenide cream (Sulfamylon), Methylene blue (Urolene Blue), Nalidixic acid (NegGram), Nitrofurantoin (Macrodantin), Phenazopyridine (Pyridium), Primaquine, Rasburicase (Elitek), Sulfacetamide (Klaron), Sulfamethoxazole (Gantanol), and Sulfanilamide (AVC).

Aspects of embodiments may include a treatment method including providing a droplet actuator, sample, and glucose-6-phosphate dehydrogenase substrate formulation, executing electrowetting-mediated droplet operations to detect glucose-6-phosphate dehydrogenase (G6PD) activity in a sample; detecting glucose-6-phosphate dehydrogenase (G6PD) activity; and administration of one or more medicaments.

4.1 Enzymatic Assays for Detection of G6PD Activity

Glucose-6-phosphate dehydrogenase deficiency is an X-linked recessive hereditary disease characterized by abnormally low levels of glucose-6-phosphate dehydrogenase (G6PD), a metabolic enzyme involved in the pentose phosphate pathway, especially important in red blood cell metabolism. G6PD deficiency is the most common human enzyme defect. Individuals with the disease may exhibit non-immune hemolytic anemia in response to a number of causes, most commonly in response to infection or exposure to certain medications, chemicals, or foods.

The G6PD enzyme catalyzes the oxidation of glucose-6-phosphate to 6-phosphoglucolactone while concomitantly reducing the oxidized form of nicotinamide adenine dinucleotide phosphate (NADP⁺) to nicotinamide adenine dinucleotide phosphate (NADPH). The NADPH produced will fluoresce under long-wave UV light (340 nm excitation/460 nm emission) during the reaction. FIG. 1 illustrates an example of a process 100 of reducing NADP⁺ to NADPH to generate a fluorescence signal. As glucose-6-phosphate is oxidized to 6-phosphogluconolactone, the coenzyme NADP⁺ is reduced to NADPH with a corresponding elevation in fluorescence.

In one example, the enzymatic assays of the invention are performed in fresh or fresh-frozen whole blood samples. An aliquot of fresh whole blood may be combined with an aliquot of extraction buffer such as 0.1% (w/v) Tween® 20 in molecular grade water and analyzed directly. Alternatively, the whole blood sample in extraction buffer may be stored at −80° C. until use. In one example, a 3.1 μL aliquot of whole blood may be diluted with 96.9 μL of extraction buffer (e.g., 0.1% (w/v) Tween® 20 in molecular grade water). The prepared blood sample may be assayed directly or stored at −80° C. until use.

In another example, the enzymatic assays of the invention are performed in dried blood spot (DBS) extracts. DBS extracts may, for example, be prepared from blood samples collected and dried on filter paper. A manual or automatic puncher may be used to punch a sample, e.g., a 3 mm punch. Each punch may be placed into a separate well of a round bottomed 96-well plate. An aliquot (e.g., 100 μL) of extraction buffer such as 0.1% (w/v) Tween® 20 in molecular grade water may be added to each well that contains a DBS punch and incubated for about 30 minutes on a plate shaker at room temperature to extract the DBS samples. Extraction buffer composition (e.g., pH, detergent concentration, salts, etc.) may be selected for performance with reagents used in specific assay protocols.

Substrate formulations for the enzymatic assays of the invention may, for example, include the substrate glucose-6-phosphate, coenzyme NADP⁺, and the inhibitor maleimide. Maleimide is an inhibitor of 6-phosphogluconate dehydrogenase, a downstream enzyme in the pentose phosphate pathway. Incorporation of maleimide in the substrate formulation inhibits further production of NADPH in a secondary reaction by endogenous 6-phosphogluconate dehydrogenase in the blood sample. An example of a substrate formulation includes 100 mM Tris HCL, pH7.8; 26 mM maleimide; 2.6 mM NADP⁺; 2.4 mM magnesium chloride; and 2.0 mM glucose-6-phosphate.

The invention provides methods for droplet-based enzymatic detection of G6PD activity and for bench-based enzymatic detection of G6PD activity. In one example, the methods of the invention include, but are not limited to, the following steps:

-   -   1. Preparing a sample, e.g. a blood sample;     -   2. Preparing a substrate formulation;     -   3. Mixing an aliquot of prepared sample (e.g., blood sample)         with an aliquot of substrate formulation;     -   4. Incubating the sample at about 37° C.; and     -   5. Reading NADPH fluorescence (e.g., 340 nm excitation/460 nm         emission) at different time intervals (e.g., t=0 to t=300         seconds).

4.1.1 Assay Protocol

The invention provides methods for enzymatic detection of G6PD activity. G6PD enzymatic activity assays may be performed using fresh blood samples, fresh-frozen blood samples, or dried blood spot (DBS) samples. The assay may be performed using a microtiter plate-based assay and microtiter plate reader (e.g., Synergy H1 plate reader). In one example, the assay for G6PD enzyme activity uses glucose-6-phosphate as substrate and detection of G6PD generated NADPH fluorescence as output. The microtiter plate reader may be heated to an incubation temperature (e.g., 37° C.) and the fluorescence signal read kinetically (e.g., t=0 to t=300 seconds; 50 second intervals). In another example, the assay for G6PD enzyme activity is based on the oxidation of glucose-6-phosphate to 6-phosphogluconate, and reduction of NADP to NADPH, in the presence of G6PD. The NADPH produced reduces tetrazolium dye (MTT) in the presence of phenazine methosulfate (PES) to produce a colored product with an absorbance peak at 565 nm.

FIG. 2 shows a plot 200 of an example of a reference assay using purified G6PD to evaluate substrate formulation for automated G6PD activity assays. The assay was performed on-bench using a microtiter plate assay and Synergy H1 plate reader. The substrate formulation was 100 mM Tris HCL, pH7.8; 26 mM maleimide (obtained from Aldrich); 2.6 mM NADP (β-NADP⁺, obtained from Sigma); 2.4 mM magnesium chloride; and 2.0 mM glucose-6-phosphate (glucose-6-phosphate dipotassium salt hydrate, obtained for Sigma). A working solution of G6PD was prepared by dissolving 200 units of purified G6PD (obtained from Sigma) in 250 μL 100 mM Tris, pH 7.8. The G6PD enzyme preparation was diluted 10,000× in 100 mM Tris pH 7.8 for the assay. The assay protocol was conducted as follows. A microtiter plate reader was warmed to an incubation temperature of 37° C. An aliquot (25 μL) of the diluted purified G6PD enzyme preparation was mixed with 25 μL of substrate mix and incubated at 37° C. Fluorescence readings were obtained kinetically at intervals of 50 seconds (i.e., over time t=0 to time t=300 seconds) at excitation 340/emission 460 at a gain of 100, offset 7 mm.

FIG. 3 shows a plot 300 of an example of a comparison of G6PD enzyme activity assays performed on-bench using fresh-frozen whole blood and DBS extract samples. Fresh-frozen whole blood sample with hematocrit 50% was prepared as described with reference to FIG. 5. A DBS extract sample was prepared by extracting a 3 mm punch of a quality control (QC) base pool (BP) dried blood sample in 100 μL of extraction buffer (0.1% (w/v) Tween® 20 in molecular grade water). The dried QC-BP sample was prepared from a pool of washed, leukoreduced human red blood cells that were adjusted with human plasma to a hematocrit of 50%. The substrate mix formulation for the enzymatic reaction was 100 mM Tris HCL, pH7.8; 26 mM maleimide; 2.6 mM β-NADP⁺; 2.4 mM magnesium chloride; and 2.0 mM glucose-6-phosphate.

The assay protocol was conducted as follows. An aliquot (25 μL) of fresh-frozen whole blood sample or DBS extract was mixed with 25 μL of G6PD substrate mix in separate wells of a 96-well microtiter plate. The reaction was incubated at 37° C. for 300 seconds and fluorescence read kinetically (t=0 to t=300 seconds; 50 second intervals) at 340 nm excitation/460 nm emission using a Synergy H1 microtiter plate reader at a gain of 100, offset 7 mm. The data show an increase in NADPH fluorescence signal over time for both fresh-frozen and DBS extract samples. An increase in fluorescence signal indicates G6PD-meditate hydrolysis of the substrate glucose-6-phosphate and reduction of NADP⁺ to NADPH. Although, the fresh-frozen blood sample shows higher enzyme activity (i.e., higher fluorescence signal) than the DBS sample, both sample types are acceptable for analysis. The lower fluorescence signal in the DBS sample may be due to some expected loss of enzyme activity during storage.

To evaluate the effect of excitation wavelength on NADPH fluorescence in the G6PD assay, the excitation wavelength was adjusted to 368 nm, an average of excitation outputs of several different instruments (R100 instruments in house). FIG. 4 shows a plot 400 of an example of the effect of excitation wavelength on NADPH fluorescence in G6PD enzyme activity assays. Blood samples (i.e., fresh-frozen whole blood and DBS extract samples), substrate mix, and assay protocol were as described with reference to FIG. 3, except NADPH fluorescence was read at 368 nm excitation/460 nm emission using a Synergy H1 microtiter plate reader at a gain of 100, offset 7 mm. The data show the fluorescence signals from fresh-frozen and DBS extract samples obtained at 368 nm excitation are comparable to the signals obtained at 340 nm excitation in FIG. 3.

To evaluate the correlation between G6PD enzyme activity and NADPH fluorescence signal in the G6PD assay, the assay was performed using whole blood samples with different hematocrits. Normal blood samples with higher hematocrits have higher levels of G6PD enzyme than normal blood samples with lower hematocrits. The rate of NADPH formation is proportional to G6PD enzyme activity in the sample. FIG. 5 shows a plot 500 of an example of the effect of hematocrit on NADPH fluorescence in a G6PD assay. Blood samples with different hematocrits were prepared from fresh whole blood. Fresh whole blood was collected in a lithium heparin tubes (i.e., two 3 mL tubes from a healthy male volunteer) and mixed gently for about 10 minutes. The hematocrit was determined to be 42% using CritSpin standard microcentrifugation protocols. Aliquots (500 μL) of the whole blood sample were placed in 1.5 mL microcentrifuge tubes and used to prepare samples with adjusted hematocrits of 30, 40, 50, 60, 70, and 80%. Hematocrit level was adjusted as follows: C₁V₁=C₂V₂; where C₁ is the current hematocrit 42%, V₁ is the volume of blood sample, C₂ is the desired hematocrit, and V₂ is the reconstituted volume. To prepare samples with hematocrits of 50 to 80%, 500 μL aliquots of whole blood were fractionated by centrifugation at 1700×g for 12 minutes at room temperature. An appropriate amount of the upper plasma layer was removed from each aliquot and reserved. An 80% hematocrit sample was prepared by removing 237.5 μL of plasma from 500 μL blood. A 70% hematocrit sample was prepared by removing 200 μL of plasma from 500 μL blood. A 60% hematocrit sample was prepared by removing 150 μL of plasma from 500 μL blood. A 50% hematocrit sample was prepared by removing 80 μL of plasma from 500 μL blood. A 40% hematocrit sample was prepared by adding 25 μL of the reserved plasma to 500 μL of unfractionated blood. A 30% hematocrit sample was prepared by adding 200 μL the reserved plasma to 500 μL of unfractionated blood. After volume adjustments, samples were briefly vortexed and then mixed for 30 minutes. An aliquot (31 μL) of each hematocrit adjusted sample was mixed with 969 μL of extraction buffer (0.1% (w/v) Tween® 20 in molecular grade water) and stored in 100 μL aliquots at −80° C. until use.

The assay protocol was conducted as follows. An aliquot (25 μL) of each hematocrit adjusted blood sample was mixed with 25 μL of G6PD substrate mix (100 mM Tris HCL, pH7.8; 26 mM maleimide; 2.6 mM β-NADP⁺; 2.4 mM magnesium chloride; and 2.0 mM glucose-6-phosphate) in separate wells of a 96-well microtiter plate. The reaction was incubated at 37° C. for 300 seconds and fluorescence read kinetically (t=0 to t=300 seconds; 50 second intervals) at 368 nm excitation/460 nm emission using a Synergy H1 microtiter plate reader at a gain of 100, offset 7 mm. The data show an increase in NADPH fluorescence signal over time with increasing hematocrit. Self-quenching of fluorescence was observed at a hematocrit of 80%. The data show NADPH fluorescence signal is directly correlated to hematocrit of the sample.

FIG. 6 shows a plot 600 of normal, intermediate, and deficient G6PD control samples screened on-bench for G6PD activity. Lyophilized control samples, normal (G6888), intermediate (G5029), and deficient (G5888), were obtained from Trinity Biotech (Bray, Ireland). Samples were reconstituted with 0.5 mL molecular grade water with gentle rotation for 30 minutes. An aliquot (3.1 μL) of each control was diluted in 96.9 μL of extraction buffer (0.1% (w/v) Tween® 20 in molecular grade water). Aliquots (25 μL; n=5) of each diluted control sample were mixed with 25 μL of G6PD substrate mix (100 mM Tris HCL, pH7.8; 26 mM maleimide; 2.6 mM β-NADP⁺; 2.4 mM magnesium chloride; and 2.0 mM glucose-6-phosphate) in separate wells of a 96-well microtiter plate. The reaction was incubated at 37° C. for 300 seconds and the fluorescence read kinetically (t=0 to t=300 seconds; 50 second intervals) at 368 nm excitation/460 nm emission using a Synergy H1 microtiter plate reader at a gain of 100, offset 7 mm. The data show a clear separation in fluorescence signal between normal, intermediate, and deficient G6PD control samples.

4.1.2 Droplet-Based Assay Protocol

The invention provides methods for a droplet-based enzymatic assay for G6PD activity. G6PD enzymatic activity assays may be performed using fresh blood samples, fresh-frozen blood samples, or dried blood spot (DBS) samples. On-bench assays for determination of G6PD activity may be adapted and described as discrete step-by-step droplet-based protocols. Assay protocol parameters may, for example, be selected for linearity, increased sensitivity (limit of detection), droplet carryover, and rapid time-to-result.

Digital microfluidic enzyme assays are performed in aqueous droplets within an oil-filled gap of a droplet actuator. Samples and assay reagents are manipulated as discrete droplets upon an arrangement of electrodes (i.e., digital electrowetting). Sample droplets and reagent droplets for use in conducting the enzymatic assays may be dispensed and/or combined according to appropriate assay protocols using droplet operations on a droplet actuator. Incubation of assay droplets, including temperature adjustments as needed, may also be performed on a droplet actuator. Further, detection of signals from assay droplets, such as detection of fluorescence, may be conducted while the droplet is present on the droplet actuator. Further, each of these processes may be conducted while the droplet is partially or completely surrounded by a filler fluid on the droplet actuator.

Certain assay steps may be conducted outside of a droplet actuator and certain assay steps may be conducted on a droplet actuator. For example, in some embodiments, samples and reagents may be prepared outside the droplet actuator and combined, incubated, and detected on the droplet actuator. In one example, samples (e.g., fresh-frozen blood samples, DBS samples) used for testing for G6PD activity are prepared using an on-bench protocol prior to loading on a droplet actuator. Reagent preparation (e.g., extraction buffer and substrate formulations) may also be prepared using on-bench protocols prior to loading on a droplet actuator. In another example, reagent and/or samples are prepared in reservoirs associated with the droplet actuator then flowed to different operations gaps, and/or prepared in the droplet operations gap.

An example of a digital microfluidic testing assay for G6PD activity was conducted as follows. A 1× sample droplet (e.g., a fresh-frozen blood sample droplet) is combined and mixed using droplet operations with a 1× G6PD substrate droplet (e.g., 100 mM Tris HCL, pH7.8; 26 mM maleimide; 2.6 mM β-NADP⁺; 2.4 mM magnesium chloride; and 2.0 mM glucose-6-phosphate) droplet to form a 2× reaction droplet. The 2× reaction droplet is transported using droplet operations to a detection spot within a temperature control zone. The temperature control zone may be set, for example, at 37° C. Fluorescence of the 2× reaction droplet is measured (t-0 seconds). The 2× reaction droplet is incubated at 37° C. for a predetermined time (e.g., 300 seconds) and fluorescence measured at one or more time points (e.g., t-50, t-100, t-150, t-200, t-250, and t-300 seconds). In this example, a single sample droplet is dispensed and analyzed.

However, any number of sample droplets may be dispensed and analyzed. G6PD activity is determined from the fluorescence signal.

An additional example of a G6PD enzyme activity assay was performed on-actuator using whole blood samples. The assay for G6PD enzyme activity was based on the oxidation of glucose-6-phosphate to 6-phosphogluconate, and reduction of NADP to NADPH, in the presence of G6PD. The NADPH produced reduced tetrazolium dye (MTT) in the presence of phenazine methosulfate (PES) to produce a colored product with an absorbance peak at 565 nm. Whole blood samples from presumed normal individuals were used. A G6PD deficiency neonatal screening test kit was obtained from Interscientific Corp (Hollywood, Fla.). The kit included the following reagents: R1, elution/lysis buffer; R2, work reagent; R3, color reagent; R4, color reagent buffer (CRB). The screening test kit also included assay controls representing normal, intermediate, and deficient values. For the assay, 1 part of CRB (R4) was combined with 10 parts color reagent (R3) to prepare a solution of working color reagent. Prepared reagents, controls and samples were loaded onto fluid dispensing reservoirs of a droplet actuator.

The on-actuator assay was conducted as follows. A 1× sample droplet was combined using droplet operations with three 1× droplets of elution/lysis buffer (R1) to yield a 4× sample droplet. The 4× sample droplet was mixed using droplet operations for 1 minute. The 4× sample droplet was split using droplet operations to yield a 1× sample droplet. The 1× sample droplet was combined using droplet operations with three additional 1× droplets of elution/lysis buffer (R1) to yield a 4× sample droplet and incubated for 1 minute. The 4× sample droplet was split using droplet operations to yield a 1× sample droplet. The 1× sample droplet was combined using droplet operations with a 1× droplet of working reagent (R2) to yield a 2× reagent/sample droplet and incubated for 30 seconds. The 2× reagent/sample droplet was split using droplet operations to yield a 1× reagent/sample droplet. The 1× reagent/sample droplet was combined using droplet operations with two additional 1× droplets of working reagent (R2) to yield a 3× reagent/sample droplet and incubated for 30 seconds. The 3× reagent/sample droplet was split using droplet operations to yield a 1× reagent/sample droplet and a 2× reagent/sample droplet. The 2× reagent/sample droplet was transported using droplet operations to a detector electrode to measure absorbance at 405 nm for hemoglobin normalization. The 1× reagent/sample droplet was combined using droplet operations with a 1× droplet of working color reagent to yield a 3× sample droplet. The 3× sample droplet was transported using droplet operations to a detector electrode to measure absorbance at 560 nm in kinetic mode. The same droplet protocol was performed for the control samples (i.e., deficient, intermediate, and normal). The sample concentration was expressed using the following formula:

${{Sample}\mspace{14mu}\left( {U\text{/}g\mspace{11mu}{HB}} \right)} = {\frac{\Delta\;{OD}_{{SAMPLE}\mspace{11mu} 560\mspace{11mu}{nm}}\text{/}\Delta\;{OD}_{{CONTROL}\mspace{11mu} 560\mspace{11mu}{nm}}}{\Delta\;{OD}_{{SAMPLE}\mspace{11mu} 405\mspace{11mu}{nm}}\text{/}\Delta\;{OD}_{{CONTROL}\mspace{11mu} 405\mspace{11mu}{nm}}}x\mspace{14mu}{Control}}$

The resulting data showed the hemoglobin-normalized absorbance values obtained for normal, intermediate, and deficient controls and eight presumed normal whole blood samples. The data also showed good separation between deficient, intermediate, and normal samples.

4.2 Systems

FIG. 7 illustrates a functional block diagram of an example of a microfluidics system 700 that includes a droplet actuator 705. Digital microfluidic technology conducts droplet operations on discrete droplets in a droplet actuator, such as droplet actuator 705, by electrical control of their surface tension (electrowetting). The droplets may be sandwiched between two substrates of droplet actuator 705, a bottom substrate and a top substrate separated by a droplet operations gap. The bottom substrate may include an arrangement of electrically addressable electrodes. The top substrate may include a reference electrode plane made, for example, from conductive ink or indium tin oxide (ITO). The bottom substrate and the top substrate may be coated with a hydrophobic material. Droplet operations are conducted in the droplet operations gap. The space around the droplets (i.e., the gap between bottom and top substrates) may be filled with an immiscible inert fluid, such as silicone oil, to prevent evaporation of the droplets and to facilitate their transport within the device. Other droplet operations may be effected by varying the patterns of voltage activation; examples include merging, splitting, mixing, and dispensing of droplets.

Droplet actuator 705 may be designed to fit onto an instrument deck (not shown) of microfluidics system 700. The instrument deck may hold droplet actuator 705 and house other droplet actuator features, such as, but not limited to, one or more magnets and one or more heating devices. For example, the instrument deck may house one or more magnets 710, which may be permanent magnets. Optionally, the instrument deck may house one or more electromagnets 715. Magnets 710 and/or electromagnets 715 are positioned in relation to droplet actuator 705 for immobilization of magnetically responsive beads. Optionally, the positions of magnets 710 and/or electromagnets 715 may be controlled by a motor 720. Additionally, the instrument deck may house one or more heating devices 725 for controlling the temperature within, for example, certain reaction and/or washing zones of droplet actuator 705. In one example, heating devices 725 may be heater bars that are positioned in relation to droplet actuator 705 for providing thermal control thereof.

A controller 730 of microfluidics system 700 is electrically coupled to various hardware components of the invention, such as droplet actuator 705, electromagnets 715, motor 720, and heating devices 725, as well as to a detector 735, an impedance sensing system 740, and any other input and/or output devices (not shown). Controller 730 controls the overall operation of microfluidics system 700. Controller 730 may, for example, be a general purpose computer, special purpose computer, personal computer, or other programmable data processing apparatus. Controller 730 serves to provide processing capabilities, such as storing, interpreting, and/or executing software instructions, as well as controlling the overall operation of the system. Controller 730 may be configured and programmed to control data and/or power aspects of these devices. For example, in one aspect, with respect to droplet actuator 705, controller 730 controls droplet manipulation by activating/deactivating electrodes.

Detector 735 may be an imaging system that is positioned in relation to droplet actuator 705. In one example, the imaging system may include one or more light-emitting diodes (LEDs) (i.e., an illumination source) and a digital image capture device, such as a charge-coupled device (CCD) camera.

Impedance sensing system 740 may be any circuitry for detecting impedance at a specific electrode of droplet actuator 705. In one example, impedance sensing system 740 may be an impedance spectrometer. Impedance sensing system 740 may be used to monitor the capacitive loading of any electrode, such as any droplet operations electrode, with or without a droplet thereon. For examples of suitable capacitance detection techniques, see Sturmer et al., International Patent Publication No. WO/2008/101194, entitled “Capacitance Detection in a Droplet Actuator,” published on Aug. 21, 2008; and Kale et al., International Patent Publication No. WO/2002/080822, entitled “System and Method for Dispensing Liquids,” published on Oct. 17, 2002; the entire disclosures of which are incorporated herein by reference.

Droplet actuator 705 may include disruption device 745. Disruption device 745 may include any device that promotes disruption (lysis) of materials, such as tissues, cells and spores in a droplet actuator. Disruption device 745 may, for example, be a sonication mechanism, a heating mechanism, a mechanical shearing mechanism, a bead beating mechanism, physical features incorporated into the droplet actuator 705, an electric field generating mechanism, a thermal cycling mechanism, and any combinations thereof. Disruption device 745 may be controlled by controller 730.

It will be appreciated that various aspects of the invention may be embodied as a method, system, computer readable medium, and/or computer program product. Aspects of the invention may take the form of hardware embodiments, software embodiments (including firmware, resident software, micro-code, etc.), or embodiments combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, the methods of the invention may take the form of a computer program product on a computer-usable storage medium having computer-usable program code embodied in the medium.

Any suitable computer useable medium may be utilized for software aspects of the invention. The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. The computer readable medium may include transitory and/or non-transitory embodiments. More specific examples (a non-exhaustive list) of the computer-readable medium would include some or all of the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a transmission medium such as those supporting the Internet or an intranet, or a magnetic storage device. Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. In the context of this document, a computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.

Program code for carrying out operations of the invention may be written in an object oriented programming language such as Java, Smalltalk, C++ or the like. However, the program code for carrying out operations of the invention may also be written in conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may be executed by a processor, application specific integrated circuit (ASIC), or other component that executes the program code. The program code may be simply referred to as a software application that is stored in memory (such as the computer readable medium discussed above). The program code may cause the processor (or any processor-controlled device) to produce a graphical user interface (“GUI”). The graphical user interface may be visually produced on a display device, yet the graphical user interface may also have audible features. The program code, however, may operate in any processor-controlled device, such as a computer, server, personal digital assistant, phone, television, or any processor-controlled device utilizing the processor and/or a digital signal processor.

The program code may locally and/or remotely execute. The program code, for example, may be entirely or partially stored in local memory of the processor-controlled device. The program code, however, may also be at least partially remotely stored, accessed, and downloaded to the processor-controlled device. A user's computer, for example, may entirely execute the program code or only partly execute the program code. The program code may be a stand-alone software package that is at least partly on the user's computer and/or partly executed on a remote computer or entirely on a remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through a communications network.

The invention may be applied regardless of networking environment. The communications network may be a cable network operating in the radio-frequency domain and/or the Internet Protocol (IP) domain. The communications network, however, may also include a distributed computing network, such as the Internet (sometimes alternatively known as the “World Wide Web”), an intranet, a local-area network (LAN), and/or a wide-area network (WAN). The communications network may include coaxial cables, copper wires, fiber optic lines, and/or hybrid-coaxial lines. The communications network may even include wireless portions utilizing any portion of the electromagnetic spectrum and any signaling standard (such as the IEEE 802 family of standards, GSM/CDMA/TDMA or any cellular standard, and/or the ISM band). The communications network may even include powerline portions, in which signals are communicated via electrical wiring. The invention may be applied to any wireless/wireline communications network, regardless of physical componentry, physical configuration, or communications standard(s).

Certain aspects of invention are described with reference to various methods and method steps. It will be understood that each method step can be implemented by the program code and/or by machine instructions. The program code and/or the machine instructions may create means for implementing the functions/acts specified in the methods.

The program code may also be stored in a computer-readable memory that can direct the processor, computer, or other programmable data processing apparatus to function in a particular manner, such that the program code stored in the computer-readable memory produce or transform an article of manufacture including instruction means which implement various aspects of the method steps.

The program code may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed to produce a processor/computer implemented process such that the program code provides steps for implementing various functions/acts specified in the methods of the invention.

5 CONCLUDING REMARKS

The foregoing detailed description of embodiments refers to the accompanying drawings, which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the present invention. The term “the invention” or the like is used with reference to certain specific examples of the many alternative aspects or embodiments of the applicants' invention set forth in this specification, and neither its use nor its absence is intended to limit the scope of the applicants' invention or the scope of the claims. This specification is divided into sections for the convenience of the reader only. Headings should not be construed as limiting of the scope of the invention. The definitions are intended as a part of the description of the invention. It will be understood that various details of the present invention may be changed without departing from the scope of the present invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation. 

The invention claimed is:
 1. A method of detecting glucose-6-phosphate dehydrogenase (G6PD) activity, the method comprising: (a) mixing an aliquot of prepared sample with an aliquot of substrate formulation, wherein the substrate formulation includes about 100 mM Tris HCL, about 26 mM maleimide, about 2.6 mM NADP+, about 2.4 mM magnesium chloride, and about 2.0 mM glucose-6-phosphate, said substrate formulation having a pH of 7.8; (b) incubating the sample at about 37° C. for a time interval; and (c) detecting glucose-6-phosphate dehydrogenase (G6PD) activity present in the sample, wherein the detecting step comprises reading NADPH fluorescence at different times within the time interval wherein the time interval is about 0 seconds to about 300 seconds.
 2. The method of claim 1 further comprising: (a) executing electrowetting-mediated droplet operations using droplets on a droplet microactuator to effect an assay; (b) combining one or more substrate formulation droplets with one or more prepared sample droplets; and (c) generating and detecting a signal which corresponds to the conversion of NADP+ to NADPH in the sample.
 3. The method of claim 1 further comprising conducting the method on a droplet that is partially or completely surrounded by a filler fluid on a droplet actuator.
 4. The method of claim 1 further comprising providing a microfluidic actuator.
 5. The method of claim 1 further comprising detecting the enzymatic conversion of NADP+ to NADPH.
 6. The method of claim 1 further comprising wherein reading NADPH fluorescence is done at 340 nm excitation/460 nm emission.
 7. The method of claim 1 further comprising wherein the time interval is from about 0 seconds to about 10 minutes.
 8. The method of claim 1 further comprising wherein the sample comprises a biological sample.
 9. The method of claim 1 further comprising wherein the sample comprises an aliquot of one or more dried blood spots.
 10. The method of claim 1 further comprising wherein the sample has been isolated from a patient less than about 30 days old at the time of sample collection.
 11. The method of claim 1 further comprising wherein the detecting step comprises detecting a signal from a droplet on a droplet microactuator.
 12. The method of claim 11 further comprising wherein the signal detected corresponds to glucose-6-phosphate dehydrogenase (G6PD) activity.
 13. A method of detecting glucose-6-phosphate dehydrogenase (G6PD) activity, the method comprising: (a) mixing sample with reactant, wherein said reactant includes about 100 mM Tris HCL, about 26 mM maleimide, about 2.6 mM NADP+, about 2.4 mM magnesium chloride, and about 2.0 mM glucose-6-phosphate, said reactant having a pH of 7.8; (b) incubating the sample for a time interval; (c) detecting glucose-6-phosphate dehydrogenase (G6PD) activity present in the sample over time t=0 seconds to time t=300 seconds; and (d) conducting the method on a droplet that is partially or completely surrounded by a filler fluid on a droplet actuator.
 14. A method to assay a reaction, the method comprising: (a) executing droplet operations using droplets on a droplet microactuator to effect an assay; (b) combining one or more reactant droplets with one or more sample droplets, wherein the reactants are about 100 mM Tris HCL, about 26 mM maleimide, about 2.6 mM NADP+, about 2.4 mM magnesium chloride, and about 2.0 mM glucose-6-phosphate, said reactants having a pH of 7.8; and (c) generating and eventually detecting a signal which corresponds to the conversion of NAD(P)+ to NAD(P)H in the sample, wherein the detecting step comprises reading NADPH fluorescence at different times starting from time t=0 seconds to t=300 seconds.
 15. A method of performing a redox reaction, the method comprising: (a) providing reactants in an aqueous droplet; (b) oxidizing or reducing the reactants, wherein the reactants are about 100 mM Tris HCL, about 26 mM maleimide, about 2.6 mM NADP+, about 2.4 mM magnesium chloride, and about 2.0 mM glucose-6-phosphate, said reactants having a pH of 7.8; and (c) generating and eventually detecting a signal which corresponds to the oxidizing or reducing step, wherein the detecting step comprises reading NADPH fluorescence at intervals of 50 seconds over time t=0 seconds to time t=300 seconds.
 16. A method of detecting glucose-6-phosphate dehydrogenase (G6PD) activity, the method comprising: (a) mixing an aliquot of prepared sample with an aliquot of substrate formulation, wherein the substrate formulation includes about 100 mM Tris HCL, about 26 mM maleimide, about 2.6 mM NADP+, about 2.4 mM magnesium chloride, and about 2.0 mM glucose-6-phosphate; (b) incubating the sample at about 37° C. for a time interval; and (c) detecting glucose-6-phosphate dehydrogenase (G6PD) activity present in the sample, wherein the detecting step comprises reading NADPH fluorescence at different times within the time interval, said time interval starting at about t=0 seconds and ending at about t=300 seconds. 